There have been synthesized various electron radiation curable resins wherein double bonds in the molecules thereof are cleaved by irradiation of electron rays to proceed crosslinking reaction and to effect curing of the resins. Electron radiation curable resins include acrylic double bond-introduced resins such as vinyl chloride-vinyl acetate copolymers, epoxy resins, polyester resins, polyurethane resins, acrylic resins and the like. Such a resin is used as, for example, a binder for a magnetic composition used in the production of a magnetic recording medium such as a magnetic tape. In this case, an electron radiation curable resin is admixed with magnetic particles together with a solvent to give a magnetic composition. The magnetic composition is coated on a non-magnetic base to give a magnetic recording medium. By irradiation of electron rays on the coated surface, crosslinking reaction proceeds to effect curing of the resin. Thereby, mechanical properties and solvent resistance of the coated surface are improved. It has been known that, according to this manner, a heat treatment step can be simplified and a product in constant quality can be obtained.
Japanese Patent Kokai No. 59-77624 discloses a magnetic recording medium wherein an electron radiation curable resin is used as the binder in the magnetic composition thereof. The electron radiation curable resin is composed of a polyester urethane acrylate having plural double bonds at the terminal ends of the molecular chain, and metal sulfonate groups which has good affinity for magnetic particles in the side chains. However, since the resin has double bonds only at the terminal ends of the molecular chain, only terminal ends of the molecular chains can be crosslinked each other by irradiation of electron rays. Accordingly, crosslink density tends to become lower as increase in the molecular weight of the resin, which results in insufficient improvement of mechanical properties and solvent resistance of the coated surface.
In order to improve such a drawback, Japanese Patent Kokai No. 60-63221 discloses an electron radiation curable resin which also has double bonds in the side chains thereof. This electron radiation curable resin is obtained by reacting a prepolymer having isocyanate groups at both terminal ends and the side chains thereof, which is obtained by reaction of a copolyester with a polyisocyanate, with a compound having active hydrogen and a double bond in the molecule thereof. However, in this case, there occurs not only introduction of double bonds into the side chains, but also formation of three-dimentional structure due to branching of polyurethane acrylate through urethane bond. Accordingly, the polyurethane acrylate obtained contains gel, which results in such a problem that the objective resin in constant quality can hardly be obtained.
Further, hitherto, there has been such a problem that, when a binder for a magnetic composition which can provide good dispersion of magnetic particles in the composition is used, in general, it is difficult to stabilize their dispersion in the composition.